Stuart McLeod, the primary school headteacher who had started metal detecting a few weeks earlier, had only found horseshoes until then. He yearned to find a good early coin, like the fistfuls regularly unearthed by his veteran metal-detecting friends Stephen Tharp, a retired chef, and joiner Shawn Miller.

It took the three of them to ease their find out of its pit in the ground. It turned out to be one of the most spectacular treasure finds of 2012, 1kg of silver fashioned into a massive jug.

The silver jug dating from the 17th century. Photograph: British Museum

It was among 990 treasure finds reported that year, mostly discovered by metal detectors, according to the annual report by the British Museum. Treasure finds must be reported by law, but there were also 74,000 less valuable but often fascinating objects reported voluntarily under the Portable Antiquities Scheme.

McLeod had a wild thought that his find might be the long lost FA Cup, stolen from a Birmingham shop window in 1895 and never recovered, but the hallmarks proved it was made in 1635, presumed buried during the civil war.

"If you had silver, it was a good time to hide it," said Dora Thornton, a curator at the British Museum. Often the experts have to work for months to identify finds, but there was no mystery about the jug. It bore the London date and the maker's hallmarks – four other pieces from the same workshop have been identified, but the name of the silversmith remains unknown – as crisply as if they had been struck yesterday.

The jug was in such immaculate condition, shining as if newly polished, that they reported it to the police as well as the archaeologists, in case it had been stolen recently. In fact, as far as anyone can tell, it has lain just below the hooves of the grazing cattle in the field at Kingston Russell, west of Dorchester, for almost 400 years.

The fact that such a valuable object was never retrieved suggests things may not have gone well for the original owners.

The jug has still to be valued, but the museum in Dorchester hopes to acquire it, with the reward being shared between the three men and the landowner.

Such showy pieces were usually paired with an equally large salver, and McLeod and his friends plan to return to the field to see if they can find it.

Other exceptional finds in 2012 included a hoard of Roman silver jewellery and coins from Knutsford in Cheshire.

Alan Bates called in the archaeologists when he found the first few silver coins. When he went on to find a broken but beautiful silver gilt trumpet-shaped brooch, they told him it was the find of a lifetime and he'd never find another: he then found the second brooch, larger and in better condition a few minutes later, and a third, which is still locked within a block of soil but may be the best of the lot.

With the British Museum preparing to launch its Vikings exhibition , an Anglo-Saxon hoard of silver pennies, brooches and strap-ends found in Norfolk prove timely. The coins were minted in the 9th century time of King Edmund, later venerated as a saint, and buried just as the repeated Viking raids began on the East Anglian coast. Edmund himself lost his head, shown on the coins, in a raid in AD 869, when he was captured, tortured, bound to a tree and decapitated. According to legend, his head was guarded by a wolf, and called out "here, here!" to those who found it.

A beautifully sculpted copper alloy Roman oil jar – treasure in all but legal definition – has been loaned by landowners to the museum in Canterbury. The experts believe it was made in France and exported to Kent, where it was found in a field by a French daytripper.

More than 920,000 objects have now been reported by the public since the Portable Antiquities scheme was launched 15 years ago. The British Museum director, Neil MacGregor, said the scheme, which was without parallel, was internationally envied and emulated.